A magnetic rubber molded article produced by vulcanizing a magnetic rubber composition containing a rubber and a magnetic power has been used in a variety of applications. In particular, one suitable application of a magnetic rubber molded article is a magnetic encoder, which is produced by magnetizing a magnetic rubber molded article. Depending on performance required to an article, various rubbers are used, and in consideration of balance among oil resistance, heat resistance and a price, a nitrile rubber is suitably used. As a magnetic powder, a ferrite magnetic powder, a rare-earth magnetic powder and the like are used in accordance to required performance, and in the light of a cost and durability, a ferrite magnetic powder is suitably used (for example, see Patent Reference Nos. 1 to 3).
For improving precision of various sensors using a magnetic encoder and reducing a size, it is strongly needed to improve magnetic properties of a magnetic rubber molded article. For this purpose, a large amount of magnetic powder is routinely added to improve magnetic properties, but an excessive amount leads to deterioration in moldability, so that there is limitation to increase of the amount added. It has been, therefore, needed to provide a magnetic rubber molded article whereby magnetic properties can be improved while ensuring moldability.
Patent Reference No. 4 has described an anisotropic bond magnet made of a ferrite powder in which a particle size distribution has a plurality of peaks, a compressed density (CD) is 3.5 g/cm3 or more and a green compact has a coercivity (p-iHc) of 2100 Oe or more. Specifically, the reference has described that a composition containing the ferrite powder and 6-Nylon is injection-molded in a magnetic field to produce an anisotropic bond magnet. The reference has described that it allows for producing a high-magnetic bond magnet exhibiting excellent fillability and orientation while keeping coercivity.
The polyamide (6-Nylon) used in Patent Reference No. 4 is of low viscosity at a high temperature higher than its melting point, and can be injection-molded at high speed even when it contains a large amount of ferrite magnetic powder. Meanwhile, a magnetic rubber molded article is produced by kneading a highly viscous magnetic rubber composition at a low temperature for a long period followed by vulcanization. It has been, however, found that when a highly viscous magnetic rubber composition containing a large amount of ferrite magnetic powder for a long period, coercivity of a magnetic rubber molded article obtained after vulcanization is considerably reduced due to applying intense shear force to the ferrite magnetic powder contained for a long period. This is a problem specific for a magnetic rubber composition, which is not significant for injection molding of a polyamide-containing bond magnet as described in Patent Reference No. 4. It is, therefore, strongly needed to provide a method for manufacturing a magnetic rubber molded article with high coercivity.